1. Field of the Invention
The present invention relates to a chopper power conversion apparatus for chopping a direct current power and thereby converting the direct current power into an alternating current power, a power conditioner using the power conversion apparatus, and a power generation system using the power conversion apparatus, more particularly to a power conversion apparatus suitable for converting a direct current power generated by direct current power sources such as a solar cell and a fuel cell into an alternating current power for utility interconnection, a power conditioner used in the power conversion apparatus, and a power generation system.
2. Description of the Related Art
In perspective of global environment protection, there are proactively ongoing projects in recent years for developing a power generation system using, for example, a solar cell and a fuel cell expected to be least disruptive to the environment. In an example of the power generation system, a direct current power generated by a direct current power source, such as a solar cell, is converted by a power conditioner into an alternating current power having a commercial frequency for utility interconnection so that the converted alternating current power is supplied to a household load connected to a commercial power system, and surplus power, if the power generated by the power source exceeds power consumption by the household load, is returned to the system as a reverse power flow.
The power conditioner used in the power generation system conventionally includes an inverter for converting the direct current power generated by the solar cell into the alternating current, and a protection device for utility interconnection. There are two types of devices as such a power conditioner; an insulator device wherein a direct current unit and an alternating current unit are electrically insulated from each other by an insulation transformer, and a non-insulator device wherein the insulation transformer is not used. Comparing these two different devices, the latter non-insulator device, which is more advantageous in its power conversion efficiency, is more often used. An example of the latter device is recited in Japanese Patent Cited Document 1 (Unexamined Japanese Patent Publication No. 2002-10496).
FIG. 17 illustrates an example of a solar photovoltaic power generation system provided with a non-insulator power conditioner. A power conditioner 36 illustrated in the drawing is operable for utility interconnection with a commercial power source 2. The power conditioner 36 is provided with a smoothing capacitor 33 for smoothing power generated by and outputted from a solar photovoltaic panel 1, an inverter 34 for PWM control, a filter 35 having a reactor and a capacitor, and a control circuit not shown.
In the power conditioner 36, the generated power outputted from the solar photovoltaic panel 1 is smoothed by the smoothing capacitor 33. The inverter 34 has switching elements 37-40, which are, for example, four MOSFET where diodes are connected in reverse parallel to each other. The power conditioner 36 controls a switching operation of the switching elements 37-40 in the inverter 34 (the switching elements are switched on and off) at such a high frequency as around 18 kHz to thereby convert the generated power of the solar photovoltaic panel 1 smoothed by the smoothing capacitor 33 into an alternating current power synchronizing with a commercial power system and outputs the converted power. The power conditioner 36 supplies the alternating current power thus converted by way of the filter 35 to a load not shown or supplies it to the system as a reverse power flow.
The PWM-control inverter 34 provided in the power conditioner 36 is required to convert such a high direct current power as about 800 V supplied from the solar photovoltaic panel 1 into an alternating current power by switching on and off the switching elements 37-40 at the high frequency, around 18 kHz. Because of the technical feature, any power conversion apparatus provided in the conventional power conditioner 36 inevitably undergoes a large switching loss resulting from the power conversion in the power conditioner 36, resulting in a poor power conversion efficiency.